This application claims priority to European Patent Application EP 01203188.6 filed Aug. 23, 2001, which document is herein incorporated by reference.
The present invention relates to lithographic projection apparatus and methods.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to any structure or field that may be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of a substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, such a pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of very small (possibly microscopic) mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. For example, the mirrors may be matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and No. 5,523,193, which are incorporated herein by reference, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g. a wafer of silicon or other semiconductor material) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system (e.g. one at a time).
Among current apparatus that employ patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. A projection beam in a scanning type of apparatus may have the form of a slit with a slit width in the scanning direction. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, which is incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake, and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The term xe2x80x9cprojection systemxe2x80x9d should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d. The radiation system as well as the projection system may include components operating according to any of these design types for directing, shaping, reducing, enlarging, patterning, and/or otherwise controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In particular, the projection system will generally comprise means to set the numerical aperture (commonly referred to as the xe2x80x9cNAxe2x80x9d) of the projection system, and the radiation system typically comprises adjusting means for setting the outer and/or inner radial extent (commonly referred to as "sgr"-outer and "sgr"-inner, respectively) of the intensity distribution upstream of the patterning means (in a pupil of the radiation system).
Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT Application No. WO 98/40791, which documents are incorporated herein by reference
Generally, in order to realize integration of an increasing number of electronic components in an IC, it is necessary to increase the surface area of an IC and/or to decrease the size of the components. For the projection system, it is desirable in particular to increase the resolution so that increasingly smaller details, or line widths, can be imaged in a well-defined way onto a target portion. Such a projection system must comply with very stringent quality requirements.
A projection system may exhibit residual aberration. In practice, the projection system is not an ideal (diffraction-limited) system; generally the projection system is an aberration-limited system. Such aberration may be due, for example, to manufacturing tolerances and generic lens design limitations. Residual aberration may comprise low-order aberrations (e.g. third-order distortion, third-order x astigmatism, third-order 45xc2x0 astigmatism, third-order x coma, third-order y coma and third order spherical aberration) as well as higher-order aberrations (e.g. fifth-order and seventh-order distortion, x and 45xc2x0 astigmatism, x and y coma, and x and y three-wave aberration). For more information about aberrations mentioned above, see, for example, the paper entitled xe2x80x9cTowards a comprehensive control of full-field image quality in optical photolithographyxe2x80x9d, authored by D. Flagello et al., Proc. SPIE 3051, pp. 672-685, 1997, which document is incorporated herein by reference.
The influence of residual aberration becomes increasingly significant with the application of newer techniques, such as phase-shift masks or off-axis illumination, to enhance the resolving power of a lithographic projection apparatus. Moreover, the low- and higher-order aberrations are not constant in time. Such variation may be due, for example, to changing environmental conditions, reversible changes as caused by lens heating, and/or ageing of components of the projection system as caused by interaction of the radiation of the projection beam with the material of said components. In order to minimize the residual aberration (e.g. intermittently during a manufacturing process), modern projection lithography apparatus generally comprise means to measure low-order and/or higher-order aberrations contributing to said residual aberration, means to adjust said aberrations (e.g. through adjustments of the position of one or more movable lens elements of the projection system, or of the support structure), and means to calculate and apply the required adjustments. For a description of a method to substantially minimize residual aberration, see, for example, European Patent Application 01303036.6, which document is incorporated herein by reference.
International Patent Application WO 00/31592, which document is incorporated herein by reference, discloses methodology for the determination of aberration in an optical projection system. In particular, this WO application describes the Aberration Ring Test (xe2x80x9cARTxe2x80x9d). This technique employs a series of ring-like features on a special test reticle, which are imaged through an optical projection system onto a photosensitive substrate. The images of the ring-like features on the substrate are then inspected, using a technique such as SEM (scanning electron microscopy). A comparison of the measured images with the corresponding original features on the reticle may reveal the type(s) of aberration that the optical projection system has introduced into the images.
The same WO application also describes a refinement of the ART technique known as ARTEMIS (ART Extended to Multiple Illumination Settings). This refinement makes use of the fact that each kind of aberration can be mathematically expressed as a specific Fourier harmonic that is a combination of a number of so-called Zernike polynomials, each with an associated Zernike aberration coefficient and weighting factor. In order to determine a number N of such Zernike aberration coefficients, the ART technique is performed at a plurality N of different groups of settings of "sgr"-outer, "sgr"-inner and NA. For simplicity, a group of settings of "sgr"-outer, "sgr"-inner and NA will be referred to hereinafter as a "sgr"-NA setting. In this way, one is able to measure the same Fourier harmonic for each of the plurality N of "sgr"-NA settings. Using a simulation program, reference values can be obtained for the above-mentioned weighting factors. In combination, this technique allows the desired set of Zernike aberration coefficients to be calculated, thus allowing quantification of the aberration concerned.
An alternative method to measure aberrations of a lithographic projection system is described in European Patent Application 01301571.4, which document is incorporated herein by reference. It concerns an in situ measurement of aberrations that is performed fast enough such as to not substantially impair the number of substrates that can be processed per unit of time. According to this method, the projection beam is patterned into a desired test pattern, and the intensity distribution of the projected aerial image of the test pattern is detected in situ using detection means incorporated in the substrate table. The position of best-focus (along the optical axis of the projection system) as well as the lateral position (in mutually orthogonal directions perpendicular to the optical axis of the projection system) of the projected aerial image of the test pattern are measured for a plurality of different "sgr"-NA settings. Based on the results of said best focus and lateral position measurements, coefficients representative of one or more aberrations of the projection system may be calculated. The method is referred to hereinafter by TAMIS (Transmission image sensing At Multiple Illumination Settings).
The test pattern is typically a segment of a periodic grating comprising lines and spaces (respectively substantially blocking and transmitting projection beam radiation), for example. Segments of such gratings wherein the width of the spaces is large compared to the width of the lines may also be used as test patterns. Typically, two test patterns with the lines and spaces arranged parallel to two corresponding, mutually orthogonal, directions (in the plane comprising the pattern) are used to enable measurement of aberrations such as, for example, x coma and y coma.
However, in spite of such measures, the intensity distribution of projected aerial images of any such grating segments may not yield substantially detectable information on the presence of specific higher-order aberrations such as, for example, three-wave aberration. Consequently, there is the problem of providing test patterns suitable for reliably indicating and measuring the presence and magnitude of both low-order and higher-order aberrations, where the measurement can be done in situ such as to not substantially impair the number of substrates that can be processed per unit of time.
Embodiments of the invention include a method of measuring aberration with improved sensitivity.
A method of determining aberration of a projection system according to one embodiment of the invention includes supplying a projection beam of radiation, using a test pattern to pattern the projection beam, and using the projection system to project the patterned beam. Such a method also includes directly measuring an aerial image of the test pattern as formed by the projection system to obtain a corresponding value of each of at least one parameter.
Based on the at least one corresponding value, at least one coefficient relating to aberration of the projection system is calculated. In this case, the test pattern includes a two-dimensional lattice comprising a plurality of unit cells, each unit cell including at least three isolated areas. At least one of a transmissivity, a reflectivity, and a phase-shifting property of the isolated areas is substantially different from that of a remainder of the area of the unit cell.
In particular applications of such a method, each unit cell may have a triangular, quadrangular, or hexagonal shape. In these or other applications, the direct measurement may be performed at each of a plurality of different illumination settings (i.e. different numerical aperture settings; different settings of the outer and/or inner extent of the intensity distribution of the projection beam; different illumination modes such as disc-shaped, annular, and quadrupolar; etc.). The direct measurement may also be performed using radiation detection means including a plurality of radiation apertures.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d, or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d, and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).